Project 3 focuses on the ability of particular neuropeptides expressed in the hypothalamus to regulate mood and motivational state. The Project focuses on three particular peptides: MSH (melanocyte stimulating hormone), orexin (hypocretin), and MCH (melanin concentrating hormone). Each of these peptides is known to be involved in the control of feeding behavior. Less appreciated, however, is the connection-both anatomical and functional-between each of these peptide systems and the brain's reward circuitry. MSH produces its central effects largely through the MC4 receptor, and this receptor is highly enriched within the nucleus accumbens (NAc). We have demonstrated abnormalities in drug reward behaviors in mice with altered levels of MC4R, an effect mediated at least partly by the NAc. We have also found abnormalities in mood regulation under these conditions. A similar situation exists for orexin and MCH. The VTA (ventral tegmental area), which provides dopaminergic input to the NAc, receives one of the richest orexin projections in the brain, where orexin (via the OX1 receptor) regulates the activity of dopamine neurons. We have found that orexin knockouts show abnormalities in mood regulation. Effects of MCH in rodents appear to be mediated via one receptor, MCH1 R, which is highly enriched within the NAc. We have documented dramatic effects of MCH in the NAc in animal models of mood regulation. Our hypothesis is that these hypothalamic peptides provide a critical link between the hypothalamus's function in consumatory behavior and the VTA-NAc's function in reward, and that these links are critical regulators of mood and motivational state. The goal of the proposed studies is to further delineate the circuitry of these peptide systems between the hypothalamus and the VTA-NAc pathway, and establish the role these peptides play in the regulation of mood and motivation in animal models of depression. We will utilize mice with mutations in these various peptides or their receptors as well as local infusions of receptor agonists and antagonists for this purpose. We also will characterize regulation of these peptides by stress and antidepressant treatments, and explore a role for CREB in mediating this regulation. Such a role is indicated by the fact that stress induces CREB in a subset of hypothalamic neurons that expresscertain of these peptides. Conversely, we will explore whether the long- term effects of these peptide systems in the NAc and VTA may be mediated in part via their regulation of CREB. Such regulation would be expected based on the known G protein and intracellular messenger coupling mechanisms of the peptide receptors.